1. Field of the Invention
The invention relates generally to nuclear reactors and, more particularly, to methods and tooling for dismantling, casking and removing nuclear reactor core structures from the containment building, for example, for on-site storage of transport off-site for disposal.
2. Background Information
As nuclear reactors age, utility companies desired to extend plant life. There is also a desire for design upgrades in some circumstances. Accordingly, the replacement of reactor internals (e.g., core structures) is coming into prominence.
To remove and replace radioactive structural members of the reactor internals efficiently and cost-effectively, a number of factors must be taken into account. Among them is the very important priority of minimizing the exposure of personnel to radiation. It is also necessary to minimize plant outage duration, and to limit the size and weight of the disposable segments of the internals. For example, to minimize costs, it is desirable that the capacity of the existing crane, which is typically present at the containment building for the nuclear reactor, is not exceeded. It is also desirable that the casked segments can exit the containment building through the existing equipment hatch. Unfortunately, prior proposals do not satisfy these criteria. Rather, they typically require the existing equipment hatch to be enlarged or an alternate opening to be provided which is sufficiently large in size, for example, by breaking through the concrete and rebar of the containment building. The building must then be restored after the task is completed, at great expense. This is because such proposals require a cask which is quite large, and thus heavy, in order to house and adequately shield the radioactive internals which are to be disposed therein. Specifically, the cask or casks which generally comprise thick walled cylinders that enclose the internals to provide the shielding function, must satisfy the allowable radiation dose level on the outer surfaces of the cask(s), as prescribed by well-established health physics guidelines. This generally results in the cask(s) having relatively thick walls, thus being large and heavy. Accordingly, a special, enlarged opening, and specialized lifting equipment, including a larger capacity crane than the existing on-site crane, are required.
By way of example, one known project wherein the upper and lower internals of Shikoku Electric Power Company's Ikata Unit No. 1 were replaced, required a single cask which was large (e.g., about 12 m in height and 3.8 m in outer diameter) and heavy (e.g., about 450 tons). Such a large cask severely limits the number of manufacturing vendors who have the necessary equipment to cast, machine and handle thick walled cylinders of the magnitude necessary. Additional disadvantages included extended material procurement and manufacturing schedules. Accordingly, such a process is cost-intensive.
There is a need, therefore, for an improved method and tooling for dismantling, casking and removing nuclear reactor core structures which overcomes the aforementioned disadvantages. Specifically, it is desirable to selectively dismantle highly radiated components and cask them in a fashion which significantly reduces the size and weight of disposal hardware.
There is, therefore, room for improvement in methods and tooling for dismantling, casking and removing nuclear reactor core structures.